1. Field of the Invention
The present invention relates to a process for producing an encapsulated photosensitive material having an extended dynamic range and improved continuous tone, and more specifically, to a process for producing such material by altering the concentration of the internal phase during encapsulation.
2. Description of the Prior Art
Imaging systems employing photosensitive microcapsules are described in U.S. Pat. Nos. 4,399,209 and 4,440,846, assigned to The Mead Corporation. These patents teach the formation of images by the exposure-controlled release of an image-forming agent from a microcapsule containing a photohardenable composition. The image-forming agent is typically a substantially colorless color precursor which reacts with a developer to form a visible image.
U.S. Pat. Nos. 4,842,976 and 4,576,891 disclose a full color imaging system wherein three sets of microcapsules which are sensitive to different bands of actinic radiation are employed. These microcapsules respectively contain cyan, magenta and yellow color precursors.
Panchromatic, full color, imaging systems wherein a panchromatic imaging sheet is exposed to visible light and subsequently developed are described in European Application No. 0233567.
Although the above-described inventions have significantly advanced imaging technology, the inventions still suffer from minor drawbacks. For example, for some photosensitive materials, the resulting H&D curve has a high gamma, causing a relatively narrow dynamic range. For applications which require or prefer continuous tone images, it is desired to extend the dynamic range of the materials.
In addition, for full color or panchromatic imaging systems, it is desirable that the systems be color balanced. Color imbalance may result when the H&D curves of respective cyan, magenta and yellow microcapsules are not properly aligned. Accordingly, it is desirable to devise a system wherein the H&D curves are aligned.
Several alternative means have been proposed to cure the above-described defects. For example, U.S. Pat. No. 4,882,259 discloses photosensitive microcapsules wherein a solid diluent, typically a wax or fatty acid or alcohol, is incorporated into the internal phase of microcapsules. The incorporation of the solid diluent extends the dynamic range of the microcapsules.
U.S. Pat. No. 4,873,168 discloses a photosensitive imaging method utilizing a heating step. In some photosensitive compositions, heating the microcapsule imaging sheet prior to exposure to actinic radiation functions to extend the dynamic range of the photosensitive microcapsules.
U.S. Pat. No. 4,840,866 discloses a microcapsule imaging system having an extended dynamic range. The dynamic range is extended by utilizing a mixture of two or more independently prepared sets of microcapsules having the same spectral sensitivity but different film speeds. To obtain different film speeds, various techniques may be used including: different concentrations of photoinitiators, different photoinitiators or different concentrations of photopolymerizable monomers, incorporating a radiation absorber in one set of the microcapsules and a different amount or no amount of absorber in the other set of microcapsules, utilizing different wall formers for preparing the respective microcapsules, or varying the size of the microcapsules.
Although the above-described methods have helped to remedy the above-described defects, they require the preparation of two or more types of microcapsules, the incorporation of additional materials into the internal phase of the microcapsules, or altering imaging process conditions. Further, undesirable photographic characteristics sometimes result. For example, when producing a composition from two sets of microcapsules having H&D curves with different gammas an unacceptable hump occasionally develops in the resulting H&D curve. Although the dynamic range of the mixture of different speed microcapsules is extended, the hump in the H&D curve introduces problems in achieving color balance.
Accordingly, it would be desirable to develop an encapsulated photosensitive composition having an extended dynamic range and improved photographic properties wherein the photosensitive composition may be produced in a single batch and wherein process conditions during imaging do not have to be altered.